1. Field of the Invention
The present invention relates to temperature sensing and, more particularly, to Resistance Temperature Detector (RTD) temperature sensing. The invention also relates to temperature sensing systems and excitation control systems employing an RTD to sense temperature.
2. Background Information
A Resistance Temperature Detector (RTD) senses temperature by providing a variable electrical resistance of a metal, which resistance changes with temperature. Platinum is the most commonly used metal for RTDs due to its stability and nearly linear temperature versus resistance relationship. Platinum also has the advantages of chemical inertness, a temperature coefficient of resistance that is suitably large in order to provide readily measurable resistance changes with temperature, and a resistance which does not drastically change with strain. Other types of RTDs include copper, nickel and nickel alloys.
The RTD""s resistance versus temperature relationship is qualified by a term known as xe2x80x9calphaxe2x80x9d. xe2x80x9cAlphaxe2x80x9d is the average percent change in resistance per xc2x0 C. of an RTD between 0xc2x0 C. and 100xc2x0 C. For a 100 xcexa9 platinum RTD, 0.00385xcexa9/xcexa9/xc2x0 C. is the most common alpha. Alpha is also referred to as the temperature coefficient of resistance.
Circuits for sensing temperature from a variable RTD resistance are well known in the art. See, for example, U.S. Pat. Nos. 5,040,724; 6,007,239; and 6,203,191.
Modem excitations systems typically measure the temperature of a thyristor heat sink assembly. Two-, three- and four-wire configurations may be employed in electrically connecting the RTD to the temperature sensing circuit. The sensed temperature, in turn, is employed by the excitation system to determine that the thyristor bridge is operating within suitable temperature design margins. Typically, a generator alarm or trip results when the temperature exceeds a predetermined threshold.
Because the thyristor bridge is directly electrically connected to a relatively high voltage source, suitable isolation is required. The isolation is conventionally provided by the RTD imbedded in an insulation material within a conductive housing.
Conventional technology encapsulates the RTD in a metal case and employs metal hardware, including springs, to apply positive pressure to the RTD in order to properly measure the temperature of the heat sink. A problem with such metal cases is that they reduce the creapage and clearance of the thyristor bridge with respect to ground and also make it difficult to obtain a high pot voltage required by the RTD.
Process connection fittings for RTDs include compression fittings, welded or spring-loaded National Pipe Tapered (NPT) fittings, or bayonet fittings. See, for example, U.S. Pat. Nos. 4,245,613; and 5,674,009.
A ferrule may be employed inside a compression fitting to provide a leak or a gas-tight seal. A ferrule is a small machined ring, which allows the compression fitting to be fixed onto the RTD when the compression fitting is tightened. Ferrules are available in nylon, Teflon(copyright) and lava (adjustable depth) brass and stainless steel (permanent depth).
As another example, when an RTD is installed with a xc2xd in.xc3x97xc2xd in. NPT fitting, the RTD is inserted into the process hole or opening, and the probe is tightened into place by turning the probe into the threaded connection.
There is room for improvement in RTD temperature sensing, and in temperature sensing systems and excitation control systems employing RTDs.
This need and others are met by the present invention, which simplifies conventional RTD assembly technology by eliminating metal components and employing an RTD within an electrically insulating body of suitable dimension, which body is inserted into an opening of a sensed device, such as a heat sink.
As one aspect of the invention, an RTD assembly comprises: a resistance temperature detector including two outputs; a pair of electrical conductors including a first end and a second end, the electrical conductors being electrically connected at the second end thereof to the outputs of the resistance temperature detector; an electrically insulating body including a closed end, a cavity and an open end, the resistance temperature detector being disposed within the cavity of the electrically insulating body and proximate the closed end thereof, the electrical conductors extending from the cavity of the electrically insulating body with the first end of the electrical conductors being external to the electrically insulating body; and a potting material disposed within the cavity of the electrically insulating body and about the resistance temperature detector and the second end of the electrical conductors.
The electrically insulating body may be made of polytetrafluoroethylene and may have a cylindrical shape.
As another aspect of the invention, an excitation control system comprises: a rectifier bridge including a heat sink having a temperature and also including a semiconductor device, the heat sink having an opening therein; an RTD assembly comprising: a resistance temperature detector including two outputs having a temperature dependent resistance therebetween, a pair of electrical conductors including a first end and a second end, the electrical conductors being electrically connected at the second end thereof to the outputs of the resistance temperature detector, an electrically insulating body including a closed end, a cavity and an open end, the resistance temperature detector being disposed within the cavity of the electrically insulating body and proximate the closed end thereof, the electrical conductors extending from the cavity of the electrically insulating body with the first end of the electrical conductors being external to the electrically insulating body, a portion of the electrically insulating body proximate the closed end thereof engaging the heat sink at the opening thereof, and a potting material disposed within the cavity of the electrically insulating body and about the resistance temperature detector and the second end of the electrical conductors; a circuit adapted to control the rectifier bridge; and a circuit receiving the first end of the electrical conductors of the RTD assembly, the circuit adapted to sense the temperature of the heat sink from the temperature dependent resistance of the resistance temperature detector.
The electrically insulating body of the RTD assembly may be made of polytetrafluoroethylene. The opening of the heat sink may be cylindrical, and the electrically insulating body may have a cylindrical shape.
As another aspect of the invention, a temperature sensing system comprises: a heat sink having a temperature and having an opening therein; an electrical device mounted on the heat sink; an RTD assembly comprising: a resistance temperature detector including two outputs having a temperature dependent resistance therebetween, a pair of electrical conductors including a first end and a second end, the electrical conductors being electrically connected at the second end thereof to the outputs of the resistance temperature detector, an electrically insulating body including a closed end, a cavity and an open end, the resistance temperature detector being disposed within the cavity of the electrically insulating body and proximate the closed end thereof, the electrical conductors extending from the cavity of the electrically insulating body with the first end of the electrical conductors being external to the electrically insulating body, a portion of the electrically insulating body proximate the closed end thereof engaging the heat sink at the opening thereof, and a potting material disposed within the cavity of the electrically insulating body and about the resistance temperature detector and the second end of the electrical conductors; and a circuit receiving the first end of the electrical conductors of the RTD assembly, the circuit adapted to sense the temperature of the heat sink from the temperature dependent resistance of the resistance temperature detector.